varian 3800 IS area Vs septa type

[danicrd]

Hi everyone
I have a 3800GC coupled with MS4000: our problem is an extremely variation in IS area during analytical batch.
We are analysing PAH with large volume injection, in dichloromethane, with db-5ms 30m*0.25*0.25: the same method on another instrument (MS320 triple quad) is working well.
The field engineer told me that septum we are using is wrong: 1079 inj need 11,5 mm septum, not 11mm.
Is it possible that problem comes from the septa? The pressure in the injector was ok, no air or water leak.
Thanks to all.
Bye

[AICMM]

If you do not see air/water in the MS I would not suspect that the septum is the problem. What internal standards are you using, do you see fluctuation in the analytes with the internal standards and how much of what solvent are you injecting? What does the IS do if you simply run a 1 uL injection over and over again? What flow rate are you running at?

Best regards.

[danicrd]

The IS are using are those suggested by EPA8270 (ie naphtalene-d8, acenaphtene-d10, phenantrene-d10, perilene-d12).
We inject 25uL of dichloromethane, using solvent venting technique, and column flow rate is 1.2mL/1'.
I think the problem can be located in the MS, not in the injector: we are using MS4000 in external configuration.
In the next days I'll change configuration to internal to exclude any oscillation
generated by lenses of source.